1. Field of the Invention
The present invention relates to a fluid coupling apparatus which can control output torque of the coupling through three stages in response to a temperature.
2. Description of the Prior Art
As to a conventional fluid coupling for driving a cooling fan of an automobile, fluid flow between a storage chamber and a working chamber is controlled in two stages in order to reduce a power loss in addition to preventing the engine from excess cooling. However, in this two stage control, because rotation of the fan is simply switched from high speed to low speed at a set temperature, the fluid coupling apparatus may cause noise and may cool the engine too much or less than expected.
In order to overcome these drawbacks, for example, in Japanese Patent Laid-Open 55(1980)-69326, a fluid coupling apparatus which controls fluid flow in three stages was developed. In such a three stage controlled fluid coupling apparatus, two slots are provided on a plate which divides the storage chamber and the working chamber for connecting both chambers. If the temperature is under a first set value, a thermal responsive member closes the two slots so that the fan is controlled to a low number of revolutions by limiting the flow of the fluid (see FIG. 5). When the temperature reaches the set value, the thermal responsive member opens one of the slots to supply the fluid into the working chamber. The fan is then controlled to a medium number of revolutions (see FIG. 5). If the temperature goes to the second set temperature, the thermal responsive member opens both of the slots in order to provide a maximum torque increase. The fan is then controlled to provide a high number of revolutions (see FIG. 5). Accordingly, the fluid coupling apparatus controls the fan revolution in three stages.
However, in the three stage fluid coupling apparatus of the prior art, as shown in FIG. 5, the fan revolutions are changed in a step by step fashion in accordance with the first and second set temperatures. Therefore, the changes in revolutions are abrupt at the set temperatures and especially at the second set temperature as fan noise is quite loud and generates an uncomfortable feeling. The revolution change also causes an abrupt torque change; therefore, the shock of the torque change adversely affects the fan belt's life. The reasons of this large fan noise is the abrupt change in the fan revolutions and the absolute level of the fan noise is large because of the high level of fan revolutions.